My completely impractical solution to my major problems with Reminders (which appears to have worked, at least for now)
I rely heavily on Reminders, and have for the past several years. For the past many months, I've been having all sorts of problems. The "done"/"ignored" status of reminder notifications was not properly syncing between computers was the first thing I noticed. Then, about the time iOS 13 came out, things took a nasty turn for the worse. It took hours for reminders to sync to and from my phone, and Reminders just became generally unreliable and almost useless. Reminders on iCloud.com was crashing for a while, but that eventually got resolved by Apple.
Note that I never upgraded my reminders. I held off on upgrading to Catalina, and refrained from upgrading my reminders while they were in such a messed up state.
I looked a little closer at my reminders and found that I had more than 10,000 completed reminders on my personal iCloud account alone. These completed reminders were 90% daily/weekly repeating reminders (to take meds, take out the trash, etc. As I said, I rely heavily on reminders.)
And here comes the completely impractical solution: Over the course of about a month, I deleted more than 9000 completed reminders. I did this from my desktop by searching for specific reminders, selecting the completed instances and hitting the delete key. At first Reminders was regularly "not responding" when I was deleting hundreds of items at a time, and sometimes I'd have to login to iCloud.com to delete reminders, but the crashing while deleting reminders eventually resolved.
I still have about 2000 completed Reminders in my Reminders app, but the performance of Reminders appears to have returned to normal. Reminders appear to sync between my phone and desktop and the cloud and I am cautiously optimistic that I can finally trust them again.
I've now put all my repeating reminders into their own list, and plan to to regular maintenance by deleting completed tasks.
Should I have had to do this? Absolutely not. Will this work for you? I have no idea. Has Apple massively screwed something up with reminders? Yes.
Of course, YMMV, but this is something you might want to look at if you are too impatient to wait for Apple to fix it.
